When was Mendocino County founded?

Mendocino County was founded in 1850, formed from Sonoma County, and named after Antonio de Mendoza.

What is the county seat of Mendocino County?

The county seat of Mendocino County is Ukiah.

Is there an online parcel map for Mendocino County?

Yes. A public source publishes a parcel map of 62,051 parcels, rated good enough here. You can click any parcel on it in Parcel Chart's free map, no sign-in needed.

Has Mendocino County's boundary or name changed?

Mendocino County was created in 1850 from Sonoma County. Since then the Atlas of Historical County Boundaries records 11 later changes, chiefly involving Sonoma County, Lake County and Trinity County.

Is my lot in a flood zone?

We do not have flood maps for Mendocino County yet. Look up the address on FEMA's Flood Map Service Center, which shows the effective flood map for any lot.

County · code 06045

Mendocino CountyCalifornia

Mendocino County is a county in California, with its seat in Ukiah. It was created on February 18, 1850, from Sonoma County and named after Antonio de Mendoza.

Founded1850
County seatUkiah
Named forAntonio de Mendoza
Land area3,507 sq mi + 371 sq mi water
Population91,601 2020 Census
Parcels mapped62,051 checked 15 October 2026

Open the free map Click any parcel in Mendocino County: acreage, parcel id, address and who publishes the map. No sign-in.

Updated 19 October 2026

Census outline · seat marked

History

Formed in 1850 from Sonoma County

An old deed may name a county that no longer exists, or a boundary that has since moved.

Mendocino County was created from Sonoma County in 1850 but was not fully organized and was attached to Sonoma for judicial purposes. Over the following decades, the county's boundaries shifted repeatedly through exchanges and transfers with neighboring counties including Napa, Humboldt, Trinity, Lake, Glenn, and Sonoma, with a major boundary dispute with Trinity County settled by the California Supreme Court in 1907.

Boundary changes from the Newberry Library, Atlas of Historical County Boundaries; the 5 that matter most are shown first.

1850
Mendocino County created Mendocino created from non-county area; Mendocino not fully organized, attached to Sonoma "for judicial purposes."
from Sonoma
18 February 1850 · Calif. Stats. 1849, 1st sess., ch. 15, sec. 16/p. 61
1852
Lost territory Mendocino lost to Napa.
to Napa
16 April 1852 · Calif. Stats. 1852, 3d sess., ch. 116/p. 192
1855
Lost territory Mendocino lost to Napa.
to Napa
4 April 1855 · Calif. Stats. 1855, 6th sess., ch. 68/p. 77
1855
Exchanged territory Mendocino exchanged with Sonoma.
with Sonoma
27 April 1855 · Calif. Stats. 1855, 6th sess., ch. 125/p. 150
1859
Gained territory Mendocino gained from Humboldt and Trinity, exchanged with Sonoma.
from Humboldt from Trinity from Sonoma
11 March 1859 · Calif. Stats. 1859, 10th sess., ch. 108/pp. 98-99

Show all 14 changes (9 more)

1850
Mendocino County created Mendocino created from non-county area; Mendocino not fully organized, attached to Sonoma "for judicial purposes."
from Sonoma
18 February 1850 · Calif. Stats. 1849, 1st sess., ch. 15, sec. 16/p. 61
1851
Boundary clarified Mendocino boundaries redefined (no change).
no change
25 April 1851 · Calif. Stats. 1851, 2d sess., ch. 14, sec. 26/pp. 178-179
1852
Lost territory Mendocino lost to Napa.
to Napa
16 April 1852 · Calif. Stats. 1852, 3d sess., ch. 116/p. 192
1855
Lost territory Mendocino lost to Napa.
to Napa
4 April 1855 · Calif. Stats. 1855, 6th sess., ch. 68/p. 77
1855
Exchanged territory Mendocino exchanged with Sonoma.
with Sonoma
27 April 1855 · Calif. Stats. 1855, 6th sess., ch. 125/p. 150
1859
Gained territory Mendocino gained from Humboldt and Trinity, exchanged with Sonoma.
from Humboldt from Trinity from Sonoma
11 March 1859 · Calif. Stats. 1859, 10th sess., ch. 108/pp. 98-99
1859
Changed Mendocino fully organized, detached from Sonoma.
Sonoma
2 May 1859 · Calif. Stats. 1859, 10th sess., ch. 108, sec. 2/p. 99; History of Mendocino County, 272-273, 283
1860
Lost territory Mendocino lost to Humboldt and Trinity.
to Humboldt to Trinity
28 April 1860 · Calif. Stats. 1860, 11th sess., ch. 340/p. 334
1864
Lost territory Mendocino lost to Lake.
to Lake
29 February 1864 · Calif. Stats. 1863-1864, 15th sess., ch. 122/p. 111
1872
Lost territory Mendocino lost to Trinity when their common boundary was surveyed; later Mendocino claimed a different boundary based on an 1891 survey. California Supreme Court upheld 1872 survey on 15 May 1907.
to Trinity
30 October 1872 · Calif. Stats. 1871, 19th sess., ch. 531/p. 766; "Trinity County v. Mendocino County," in Calif. Rpts., 151: 279-289
1907
Gained territory Mendocino gained from Lake, lost to Glenn.
from Lake from Glenn
8 March 1907 · Calif. Stats. 1907, 37th sess., ch. 113/p. 135
1909
Boundary clarified Boundary of Lake with Mendocino and Colusa redefined (no change).
with Lake with Colusano change
13 March 1909 · Calif. Stats. 1909, 38th sess., ch. 211/pp. 326-328
1917
Exchanged territory Mendocino exchanged with Sonoma.
with Sonoma
30 July 1917 · Calif. Stats. 1917, 42d sess., ch. 725/p. 1396
1917
Exchanged territory Mendocino exchanged with Lake.
with Lake
31 July 1917 · Calif. Stats. 1917, 42d sess., ch. 779/pp. 1635-1640

Land and parcels

What the maps cover in Mendocino

Who publishes each layer, how good it is, and when we last checked.

Parcels Good enough

Lot lines and tax-map numbers for 62,051 parcels. Drawn for tax purposes: usually within a few feet of a survey, but not a survey.
edited 4 August 2026 · checked 15 October 2026
Address points Missing

No public address points layer found for Mendocino County yet.
Buildings Reference

Roof outlines traced from aerial imagery from FEMA Response Geospatial Office with Oak Ridge National Laboratory (ORNL). Good for seeing what is on a lot; not accurate enough to set a fence from.
checked 18 October 2026

Official: from the office that keeps the record
Good enough: a reliable statewide or national copy
Fallback: a guide only; check the official record
Missing: nothing public found yet

People and land

Mendocino by the Census

Population

91,601 2020 Census count

Housing units

41,370 2020 Census count

Land area

3,507 square miles

Water area

371 square miles

People per sq mi

26 population ÷ land area

Source: US Census Bureau, 2020 Census; US Census Bureau, 2020 Gazetteer Files. Not endorsed or certified by the Census Bureau.

Rules and offices

Mendocino County's official site

We have found the county's site but not its offices yet; start there.

County government

County government
The official website of Mendocino County.
mendocinocounty.org

See your own lot on these maps

Open Mendocino County on Parcel Chart's free map and click your parcel. Save it with a free account; the full record of your place (site map, projects, documents) is Pro, in early access.

Open the free map no sign-in Pro early access

Questions

Common questions about Mendocino County

When was Mendocino County founded?: Mendocino County was founded in 1850, formed from Sonoma County, and named after Antonio de Mendoza.
What is the county seat of Mendocino County?: The county seat of Mendocino County is Ukiah.
Is there an online parcel map for Mendocino County?: Yes. A public source publishes a parcel map of 62,051 parcels, rated good enough here. You can click any parcel on it in Parcel Chart's free map, no sign-in needed.
Has Mendocino County's boundary or name changed?: Mendocino County was created in 1850 from Sonoma County. Since then the Atlas of Historical County Boundaries records 11 later changes, chiefly involving Sonoma County, Lake County and Trinity County.
Is my lot in a flood zone?: We do not have flood maps for Mendocino County yet. Look up the address on FEMA's Flood Map Service Center, which shows the effective flood map for any lot.

Nearby